Catalyst-carrying filter

ABSTRACT

A catalyst holding filter reducing a pressure loss. A catalyst holding filter ( 10 ) is made of a ceramic support ( 4 ) of SiC covered with a catalyst layer ( 2 ). An average pore size of the ceramic support is 10-250 μm, and a porosity of the ceramic support is 40-80%. The catalyst layer contains a catalyst, a cocatalyst and a support material.

TECHNICAL FIELD

[0001] This invention relates to a catalyst holding filter for purifyingan exhaust gas of an engine. More particularly, the invention relates toa catalyst holding filter capable of efficiently conducting oxidationremoval of carbon monooxide (CO) and hydrocarbon (HC) and reduction ofnitrogen oxide (NOx) included in an exhaust gas.

BACKGROUND ART

[0002] Heretofore, as a catalyst holding filter for the purification ofan exhaust gas in an automobile, there is, for example, one of purifyingan exhaust gas of a diesel engine. As shown in FIG. 17(a) and FIG.17(b), a catalyst holding filter 100 is used by forming cells 101 as apath of an exhaust gas from a porous silicon carbide sintered body in ahoneycomb shape and alternately clogging these cells 101. It is commonthat the catalyst holding filter 100 is connected to an exhaust side ofa diesel engine and particulates (PM: particulate matter) deposited onthe filter 100 and HC, CO and the like are decomposed through oxidation.By such an oxidation decomposition is reduced pressure loss due to thepresence of the filter, whereby the exhaust gas can be purified withoutapplying a load to the engine and causing the engine stop.

[0003] Such a catalyst holding filter 100 is disclosed in JP-A-5-23512.That is, there is reported a filter wherein an average value of a poresize as measured by a mercury pressure process is within a range of 1 μmto 15 μm and a standard deviation of pore size distribution representedby common logarithm of the pore size is not more than 0.20.

[0004] By using this filter can be obtained a high catching efficiency.However, when the catalyst amount is increased for raising combustionreactivity of soot, it is confirmed that pressure loss becomesunexpectedly higher when the soot is caught over a long time.

DISCLOSURE OF THE INVENTION

[0005] The invention has noticed at problem points existing in such aconventional technique. An object thereof is to provide a catalystholding filter being small in the pressure loss of the exhaust gas. Andalso, another object is to provide a catalyst holding filter capable ofenhancing a collecting efficiency of particulates included in theexhaust gas.

[0006] In order to solve the above subject, the inventors have madevarious studies and found an unexpected phenomenon that the pressureloss is influenced by the pore size and porosity and even when the poresize and the porosity is made large, if the catalyst is coated, thepressure loss becomes rather larger.

[0007] In the invention described in claim 1 or 2, a catalyst holdingfilter for purification of exhaust gas lies in that a catalyst coatlayer comprising a catalyst, a cocatalyst and a support material isformed on particles of a ceramic support having an average pore size of10-250 μm and a porosity of 40-80% or 40-70%.

[0008] Moreover, the formation of the catalyst coat on the particles ofthe ceramic support means that the ceramic particles are sintered toform a three-dimensional network structure (ceramic support) and whenthe three-dimensional network structure is cut at two-dimensionalsection, the catalyst coat is formed around the particles other thanjoint portion of the particles constituting the ceramic filter.

[0009] In the invention of claim 3, the catalyst in the catalyst holdingfilter described in claim 1 or 2 contains an element selected from noblemetal element, element of Group Via of the periodic table and element ofGroup VIII of the periodic table.

[0010] In the invention of claim 4, the cocatalyst in the catalystholding filter described in claims 1-3 is at least one element selectedfrom cerium, lanthanum, barium and calcium or a compound thereof.

[0011] In the invention of claim 5, the support material in the catalystholding filter described in any one of claims 1-4 contains at least oneselected from alumina, zirconia, titania and silica.

[0012] In the invention of claim 6, the ceramic support in the catalystholding filter described in any one of claims 1-5 is silicon carbide,silicon nitride, cordierite, mullite, sialon, silica or zirconiumphosphate.

[0013] In the invention of claim 7, the ceramic support in the catalystholding filter described in any one of claims 1-6 has a honeycombstructure having plural through-holes defined by cell walls.

[0014] In the invention of claim 8, the ceramic support in the catalystholding filter described in claim 7 has a checkered pattern formed byalternately sealing both end portions with sealing bodies.

[0015] In the invention of claim 9, the average pore size in thecatalyst holding filer described in any one of claims 1-8 is measured bya mercury pressure process and a standard deviation (SD1) of pore sizedistribution when the pore size is represented by a common logarithm isnot more than 0.40.

[0016] In the invention of claim 10, the average pore size in thecatalyst holding filer described in any one of claims 1-9 is measured bya mercury pressure process and a standard deviation (SD1) of pore sizedistribution when the pore size is represented by a common logarithm isnot more than 0.20.

[0017] The function of the invention will be described below.

[0018] According to the invention described in claims 1 and 2, thecatalyst coat layer is formed on the surfaces of the particlesconstituting the ceramic support. When the average pore size is lessthan 10 μm, the pressure loss considerably increases because the poresize is decreased by the catalyst coat layer. And also, as the averagepore size becomes larger, the increase of the pressure loss by thecatalyst coat layer is suppressed. However, when soot is caught, if thepore size exceeds 250 μm, the pore size becomes too large, and the sootdeposited on the surface of the ceramic support inserts into the insideof the ceramic wall through such large pores to clog a gas passingportion thereof, so that it is considered that the pressure lossinversely increases irrespectively of a larger pore size. On the otherhand, when the porosity is less than 40%, the gas passing portion isclogged with the catalyst coat layer and hence the pressure loss becomeslarger. While, when it exceeds 70%, the catalyst coat layer is apt to bethickly adhered, but is easily peeled off and hence the peeled catalystcoat layer is deposited in the pores to increase the pressure loss. Ifit further exceeds 80%, the pressure loss more increases and there in nopractical use.

[0019] Moreover, when the catalyst is applied to the inner wall face ofthe ceramic support as described in JP-A-5-23512 (FIG. 18), if the sootis caught over a long time, the deposited layer of the soot is formed onthe surface and hence the pressure loss becomes larger.

[0020] According to the invention, however, the pressure loss is notbecome large even if the soot is caught over a long time, so that thefilter according to the invention can be used over a long time and isexcellent in the practical use.

[0021] According to the invention of claim 3, the deterioration throughpoison (lead poison, phosphorus poison, sulfur poison) can be decreasedand also the thermal degradation can be made small, so that thedurability of the catalyst holding filter can be improved.

[0022] According to the invention of claim 4, the cocatalyst selectedfrom at least one element of cerium, lanthanum, barium and calcium or acompound thereof is used, so that the durability of the catalyst can beimproved.

[0023] According to the invention of claim 5, the support material ismade of at least one selected from alumina, zirconia, titania andsilica. Therefore, it is possible to promote separating-off of sulfurcomponent obstructing the activity of the catalyst from the ceramicsupport. Particularly, when the catalyst holding filter is used forpurifying the exhaust gas from the diesel engine, a great amount ofsulfur component is included in a fuel, so that it is effective to usethe above oxides in the ceramic support.

[0024] According to the invention of claim 6, the ceramic support iseither of silicon carbide, silicon nitride, cordierite, mullite, sialon,silica and zirconium phosphate, so that the catalyst holding filterhaving excellent heat resistance and thermal conductivity can beprovided.

[0025] According to the invention of claim 7, the ceramic support has ahoneycomb structure, so that a contact area capable of contacting theexhaust gas with the catalyst becomes large. As a result, thepurification performance can be improved.

[0026] According to the invention of claim 8, both end portions of theceramic support are alternately sealed with sealing members in acheckered pattern, so that the exhaust gas invaded from one end of theceramic support always passes through the cell wall to discharge outfrom the other end thereof. Therefore, the purification performance canbe more improved.

[0027] According to the inventions of claims 9 and 10, when the standarddeviation of pore size distribution exceeds 0.40, or when the pore sizeis not distributed within a narrow range and the size is scattered, ifthe catalyst is coated, the slurry holding the catalyst easily collectsin a finer portion than a coarse portion through a capillary phenomenon.Therefore, there is more caused the difference in the pore size. As aresult, the gas passing path in the filter wall is ununiformized by thecatalyst coat layer and soot passes through a portion having a largersize. To this end, the catching efficiency lowers.

[0028] Furthermore, the standard deviation of the pore size distributionis desirable to be 0.05-0.40. In this case, the strength when thecatalyst is coated onto the ceramic particle is highest (FIG. 19(c)).

[0029] When the standard deviation is less than 0.05, the pore sizebecomes too uniform and hence the crack easily proceeds and the fracturestrength lowers. While when the standard deviation exceeds 0.40, thelarge pores are existent and hence the fracture strength considerablylowers.

[0030] Moreover, such a relation between the fracture strength and thepore size distribution is a peculiar problem produced when the catalystis coated onto the ceramic particles.

[0031] Such a problem is not caused when the catalyst is simply coatedonto the wall surface of the ceramic support.

[0032] However, it is considered that when the catalyst layer is formedon the ceramic particles, if the temperature is raised to about 300° C.,since the difference in thermal expansion coefficient between thecatalyst coat layer such as catalyst metal, alumina or the like andceramic being catalyst support is large, the catalyst coat layer isthermally expanded to enlarge the distance between the particles tothereby cause the crack. For this end, when the distribution of the poresize is uniform, the temperature of the filter rises in use and hencethe crack grows and the lowering of the strength is caused. And also,when the pore distribution is large, the lowering of the strengthbecomes conspicuous at a portion having a large pore size and also thestrength lowers. Therefore, when the catalyst layer is formed on theceramic particles, the difference in the strength is caused by thestandard deviation of the pore size.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a schematic view of an embodiment of the catalystsupport according to the invention.

[0034]FIG. 2 is an enlarged perspective view of a part of the catalystsupport.

[0035]FIG. 3 is a conceptional view of an embodiment of the alumina thinfilm.

[0036]FIG. 4 is a diagrammatic view of a pressure loss property.

[0037]FIG. 5 is a photograph showing a structure of SiC particle in thecatalyst support.

[0038]FIG. 6(a) is a diagrammatic view showing a state that soot iscaught on a surface layer in case of smaller pore size. FIG. 6(b) is adiagrammatic view showing a state that soot is caught in a deep layer incase of adequate pore size. FIG. 6(c) is a diagrammatic view showing astate that the pore size is large and hence soot is bridged between theinner faces. FIG. 6(d) is a diagrammatic view showing a state that thecatalyst coat layer is existent on a surface layer in case of a lowporosity. FIG. 6(e) is a diagrammatic view showing a state that thecatalyst coat layer is peeled off to laminate on pores in case of a highporosity. FIG. 6(f) is a diagrammatic view showing a state that the poresize is scattered and the catalyst is aggregated. FIG. 6(g) is adiagrammatic view showing a state that the catalyst is uniformlydispersed without scattering the pore size.

[0039]FIG. 7 is a comparative graph of a heat resistance in Test Example1.

[0040]FIG. 8 is a comparative graph of a pressure loss property in TestExample 2.

[0041]FIG. 9 is a comparative graph of heat resistances of alumina thinfilm and catalyst in Test Example 2.

[0042]FIG. 10 is a comparative graph of combustion property of soot inTest Example 2.

[0043]FIG. 11 is a comparative graph of THC, CO purification property inTest Example 2.

[0044]FIG. 12 is a diagrammatic view showing a mechanism of improvingoxidation rate by addition of CeO₂.

[0045]FIG. 13 is a comparative graph of oxidation property of sootaffecting regeneration property of DPF.

[0046]FIG. 14 is a comparative graph regeneration (combustion) rateaffecting regeneration property of DPF.

[0047]FIG. 15 is a comparative graph of a regeneration ratio of DPF.

[0048]FIG. 16 is a graph showing states of pore size distribution of afilter in Example and Comparative Example.

[0049]FIG. 17 is a schematic view of a catalyst support in theconventional technique.

[0050]FIG. 18 is a conceptional view of a wash-coat alumina layer.

[0051]FIG. 19(a) is a graph showing results of Example between pore sizeporosity and pressure loss. FIG. 19(b) is a graph showing betweenstandard deviation of pore size distribution taken by a naturallogarithm and catching efficiency. FIG. 19(c) is a graph showing betweenstandard deviation and average bending strength based on the catalystcoat,

BEST MODE FOR CARRYING OUT THE INVENTION

[0052] An embodiment of the invention will be explained in detail withreference to the accompanying drawings below.

[0053] As shown in FIGS. 1-3, a catalyst holding filter 10 according toan embodiment of the invention comprises a silicon-containing ceramic,for example, a ceramic support 15 made of a porous silicon-containingceramic sintered body silicon-containing ceramic being silicon carbideas a preferable embodiment. In the ceramic support 15 are cell walls 12.Surfaces of sintered SiC particles 4 constituting the cell wall 12 areindividually covered with a catalyst coat layer 2 at a given thickness.

[0054] The catalyst coat layer 2 comprises a catalyst and a cocatalystheld on a support member. In this embodiment, the support member is athin film 3 made of alumina (Al₂O₃) (hereinafter referred to as aluminathin film). In addition to alumina, it may be optionally changed unlessat least one selected from zirconia (zirconium dioxide: ZrO₂), titania(titanium oxide: TiO₂) and silica (silicon oxide: SiO₂) is included.

[0055] Concretely, there is ZrO₂, TiO₂ or SiO₂ as one kind of theoxides. As two kinds of the oxides is Al₂O₃/ZrO₂, Al₂O₃/TiO₂,Al₂O₃/SiO₂, ZrO₂/TiO₂ or ZrO₂/SiO₂. As three kinds of the oxides isAl₂O₃/ZrO₂/TiO₂, Al₂O₃/ZrO₂/SiO₂, Al₂O₃/TiO₂/SiO₂ or ZrO₂/TiO₂/SiO₂. Asfour kinds of the oxides is Al₂O₃/ZrO₂/TiO₂/SiO₂.

[0056] As the silicon-containing ceramic support 15, there may be usedones obtained by milling ceramic powder belonging to an oxide ceramicsuch as silicon carbide powder, silicon nitride powder or an oxideceramic such as sialon, mullite, cordierite or the like with an organicbinder, a lubricant, a plasticizer and water and extrusion-shaping andsintering. Thus, there is formed a wall flow honeycomb type-filter asshown in FIG. 1(a), FIG. 1(b) and FIG. 2.

[0057] The invention is described with reference an example of using SiCsintered body as the silicon-containing ceramic support 15.

[0058] The ceramic support 15 is constructed with SiC sintered bodywherein cells 11 as plural through-holes are regularly formed in itsaxial line direction in approximately square form at section. Thesecells 11 are separated from each other through cell walls 12. Openingportions of each cell 11 are sealed at one end face side with a sealingmember 14 and opened at the other end face side of the cell 11.Therefore, the opening portions and the sealed portions as a whole arealternately arranged at each end face side so as to indicate so-calledcheckered pattern. In the ceramic support 15 made from the SiC sinteredbody are formed many-cells 11 each having a square form at section. Inother words, the ceramic support 15 has a honeycomb structure.

[0059] Moreover, a density of the cell 11 is 200-350 cells/square inch.That is, about a half of many cells 11 are opened at upstream side endfaces and the remaining cells are opened at downstream side end faces. Athickness of the cell wall 12 partitioning the cells 11 is set to beabout 0.4 mm.

[0060] The ceramic support 15 made of the SiC sintered body has astructure of partitioning with porous cell walls 12 as shown in FIG.3(a). As shown by a curve C1 in a graph of FIG. 16, the ceramic support15 is necessary to have an average pore size m1 of 10-250 μm as measuredby a mercury pressure process. The average pore size m1 is preferablywithin a range of 10-150 μm or 10-100 μm.

[0061] When the cell wall 12 has such a pore size, it is preferable tocatch fine particulates. That is, when the average pore size m1 of thecell wall 12 is set to the above range, diesel particulates included inthe exhaust gas can be caught at a low pressure loss. When the averagepore size is less than 10 μm, as shown in FIG. 6(a), the pore size ismade smaller by the catalyst coat layer 2 and hence the pressure loss isconsiderably increased to cause the stop of the engine. And also, as theaverage pore size becomes large, the increase of the pressure loss dueto the catalyst coat layer 2 is suppressed. However, as shown in FIG.6(b), when soot 16 is caught, soot 16 deposits even in pores inside thewall when the pore size exceeds 50 μm (deep layer filtration).Furthermore, as shown in FIG. 6(c), when the pore size exceeds 250 m,the soot aggregates and soot 17 bridged between the pores of the wallare pointedly existent in the wall. The soot 16 is secondary particulateaggregate of carbon fine particles degrading permeation of gas and hencethe pressure loss becomes very high when passing through the soot layer.Therefore, it is possible to control the increase of the pressure losswhen the average pore size is set to be the above range.

[0062] As shown in FIG. 6(d), when the porosity is less than 40%, thesupport is densified by the catalyst coat layer 2 required at minimum toincrease the pressure loss. On the other hand, as shown in FIG. 6(e), asthe porosity becomes high, the region occupied by the catalyst coatlayer 2 increases, but the surface area of the particles is less and thethickness of the catalyst coat layer 2 becomes thick and resistance topeeling lowers. As a result, when the porosity exceeds 70%, the adhesionstrength durable to the resistance to gas passing near to the catalystis not maintained and the catalyst coat layer 2 is peeled off and thepeeled catalyst 18 deposited in the pores to increase the pressure loss.When the porosity exceeds 80%, this tendency is further large and thepore is partially clogged with the peeled catalyst 18 to bring about theconsiderable pressure loss.

[0063] As shown in the graph of FIG. 16, a standard deviation D1 of poresize distribution when the pore size is represented by a commonlogarithm is required to be not more than 0.40. Moreover, the standarddeviation D1 is preferably not more than 0.30, particularly not morethan 0.20. Moreover, the degree number in FIG. 16 is shown by logdifferential pore volume V1 d(n). That is,

V 1 d(1)=0,

V 1 d(n)={V(n)−V(n−1)}/{log[D(n−1)]−log[D(n)]}(n≧2)

[0064] wherein D(n) is a diameter of a fine pore and V(n) is anestimated pore volume.

[0065] When the standard deviation of the pore size distribution is notless than 0.40, or when the pore size is scattered without distributingin the limited narrow range, if the catalyst is coated, as shown in FIG.6(f), the slurry holding the catalyst is easily collected in a finerportion of the pores rather than a coarse portion thereof through acapillary phenomenon. As a result, there is more caused a difference inpores. Therefore, the path of flowing the gas in the filter wall isununiformized by the catalyst coat layer and hence the soot 16 easilypasses through pores having a large size. Consequently, the catchingefficiency is degraded. That is, as shown in FIG. 6(g), when the numberof pores satisfying a catching preferable range of the standarddeviation of not more than 0.40 is relatively large, the catchingefficiency is improved. And also, when the standard deviation is set tobe not more than 0.20, the catching efficiency is more improved and thereaction of the catalyst becomes better.

[0066] Furthermore, the standard deviation of the pore size distributionis desirable to be 0.05-0.40. Because the strength when the catalyst iscoated on the ceramic particles is highest (FIG. 19(c)).

[0067] When the standard deviation is less than 0.05, the pore size istoo uniform and the crack easily progresses due to temperature risingand the fracture strength lowers. On the other hand, when the standarddeviation is more than 0.40, large pores can be obtained and thefracture strength considerably lowers. That is, when the catalyst layeris formed on the ceramic particles, if the temperature is raised to 300°C., it is considered that the catalyst coat layer of catalyst metal,alumina and the like expands to enlarge a space between the particles tothereby cause crack because there is a difference in thermal expansioncoefficient to the catalyst support. Therefore, when the distribution ofthe pore size is uniform, the temperature of the filter in use rises andhence the crack progress and the strength lowers. And also, when thepore distribution is large, the lowering of the strength becomesremarkable in the portion having a large pore size and the strengthlowers. Therefore, when the catalyst layer is formed on the ceramicparticles, the difference in the strength is caused.

[0068] As mentioned above, the average pore size of the ceramic support15 is set to 10-250 μm and porosity of the ceramic support 15 is set to40-80%, so that the pressure loss can be lowered but also the mechanicalstrength can be improved. In addition, the catching efficiency ofparticulate included in the exhaust gas can be enhanced.

[0069] In case of producing such a ceramic support 15, there arecompounded and used, for example, silicon carbide powder having anaverage particle size of about 10-300 μm, silicon carbide powder havingan average particle size of about 0.1-5 μm, and if necessary, siliconcarbide powder having a middle average particle size as a startingmaterial, and about 6-50 parts by weight of methylcellulose as a binderbased on 100 parts by weight of ceramic powder, about 0-50 parts byweight of a pore forming agent made of a substance dissipating by heatat a stage before arriving to a sintering temperature of the ceramic forforming given porosity and pore size based on 100 parts by weight of theceramic powder and 10-50 parts by weight of a dispersion of an organicsolvent and water based on 100 parts by weight of the ceramic powder.

[0070] Then, such compounded starting materials are mixed and milled andshaped into a honeycomb through an extrusion shaping, and a part of theresulting cells are sealed in a checkered pattern. Next, the shaped bodyis dried at 100-200° C., degreased at 300-500° C. and fired in an inertatmosphere at 1800-2400° C. for 4-30 hours to obtain a desired ceramicsupport 15.

[0071] The ceramic support 15 having relatively large average pore sizeand porosity according to the embodiment of the invention can beprepared, for example, by the following method.

[0072] That is, the pore forming agent made of the substance dissipatingby heat at a stage before arriving to the sintering temperature of theceramic is previously added to the shaped body made of the ceramicstarting materials and the firing is carried out at such a state. As aresult, there can be obtained a porous sintered body having large poresin its matrix.

[0073] When the firing is carried out at a state of adding the poreforming agent, the pore forming agent is dissipated by heat at a stagebefore arriving at the sintering temperature of the ceramic and a largepore is formed in a place corresponding to the presence of the poreforming agent. Therefore, the large pore having desired size and shapecan be formed relatively simply and surely. Moreover, the pore formingagent is dissipated and hardly remains in the texture of the ceramicsupport 15. Therefore, this method has a merit that the degradation ofthe properties in the sintered body due to the incorporation ofimpurities can previously be prevented.

[0074] It is favorable that the pore forming agent is added at a stageof preparing the ceramic starting material and uniformly milled with theother materials. The pore forming agent is favorable to be concretely asubstance dissipating by heat at a stage before arriving at thesintering temperature of silicon carbide (about 2200° C.). The term“dissipating by heat” means that the substance is substantially lostfrom the sintered body by sublimation, evaporation, decomposition,reaction sintering or the like through heat. The dissipating temperatureis desirable to be low, and is concretely not higher than 1000° C. andparticularly preferable to be not higher than 500° C. As the dissipatingtemperature becomes lower, a probability of retaining the impurity inthe ceramic support 15 is less and contributes to the improvement ofsilicon carbide ratio. However, the pore forming agent is desirable notto accompany foaming in the dissipation. Because, in case of the poreforming agent accompanying the foaming, it is difficult to form largepores having uniform size and shape, which has a fear of influencingupon the quality of the ceramic support 15.

[0075] As a preferable example of the pore forming agent, there areparticles of synthetic resin and the like. In addition to this example,particles made of an organic polymer such as starch and the like may beused. In this embodiment, a spherically granular pore forming agent madeof the synthetic resin is used.

[0076] The merit of using the particles of the synthetic resin as thepore forming agent lies in a point that it is surely dissipated by heatat a relatively premature stage before arriving at the sinteringtemperature of silicon carbide. And also, the synthetic resin iscomprised of a relatively simple molecule structure and is small in thepossibility of producing a complicated compound by heating and henceimpurities resulting in the degradation of the properties of thesintered body hardly remain in the ceramic support 15. Furthermore, thesynthetic resin is a relatively cheap material, and it does not bringabout the increase of the production cost of the ceramic support 15.

[0077] Moreover, the shape of the pore forming agent is not limited tothe sphere but may be, for example, an elongated sphere, a cube, anindefinite lump, a column, a plate or the like. The average particlesize of the pore forming agent may be set to the objective pore size,for example, about 5 μm-250 μm. And also, as to the adjustment of theporosity, the pore forming agent dissipating by firing is added to thestarting material at an amount required from a calculation ofdissipation volume for a desired porosity.

[0078] As a method of holding the catalyst, it is not particularlylimited to the following method, but it is effective to cover thesurface of the cell wall 12 substantially constituting the ceramicsupport 15, particularly the surface of each sintered SiC particle 4constituting the cell wall 12 with the rare earth oxide-containingalumina thin film 3. More accurately, the surfaces of the SiC particles4 in the SiC sintered body constituting the cell wall 12 areindividually covered with the rare earth oxide-containing alumina thinfilm 3 by various methods.

[0079] Moreover, FIG. 17(b) shows such a conventional technique that thecatalyst coat layer 2 is uniformly covered and formed onto the surfaceof the cell wall 12 by a wash-coat method, while FIG. 3(a) and FIG. 3(b)are diagrammatic views of embodiments of the ceramic support 15according to the invention. The latter shows a state that the surfacesof the SiC particles constituting the cell wall 12 are individuallycovered with the rare earth oxide-containing alumina thin film 3(hereinafter abbreviated as alumina thin film 3).

[0080] Thus, the above characteristic structure in the catalyst holdingfilter 10 according to the invention is different from the conventionaltechnique wherein the wall face of the cell wall 12 for the exhaust gasis simply and uniformly covered with the catalyst coat layer 2. Forexample, when the cell wall 12 is uniformly covered as in theconventional technique, gaps between SiC particles 4 is clogged andsealed to obstruct permeation. On the contrary, the ceramic support 15used in the embodiment of the invention has a structure that eachsurface of the SiC particles constituting the cell wall 12 isindividually covered with the alumina thin film 3.

[0081] Therefore, in the embodiment of the invention, the pores of thecell wall 12 itself, i.e. spaces produced between the SiC particles 4are not completely clogged and the pores are maintained as they are. Asa result, the pressure loss can considerably be made small as comparedwith the conventional catalyst coat layer 2. And also, the alumina thinfilm 3 individually covers each SiC particle 4 itself, so that there isno peeling of the thin film from the cell wall 12 in the washing withwater. As a result, the washing resistance can be improved. Furthermore,the contacting area between the exhaust gas and the catalyst is madelarge. As a result, oxidation of CO, HC in the exhaust gas can bepromoted.

[0082] Now, the pressure loss property, heat resistance, washingresistance and regeneration property of the alumina thin film 3 aredescribed below.

[0083] (Pressure Loss Property of Alumina Thin Film 3)

[0084] In general, the pressure loss property when the exhaust gaspasses through the cell wall 12 is considered as follows. That is, thepressure loss when the diesel exhaust gas passes through the ceramicsupport 15 can be shown in FIG. 4. In this case, resistances ΔP1, ΔP2,ΔP3 are dependent upon the cell structure of the filter, respectively,and are a constant value Δpi=(ΔP1+ΔP2+ΔP3) irrespectively of the lapseof time such a deposition of diesel particulates and the like, which iscalled as an initial pressure loss. On the other hand, ΔP4 is aresistance when passing through deposited diesel particulates and is avalue higher than 2-3 times the initial pressure loss.

[0085] Since a specific surface area of the ceramic support 15 having acell structure of 14/200 is 8.931 cm²/cm³ and a density of the ceramicsupport 15 is 0.675 g/cm³, a specific surface area of the cell wall 12is 0.0013 m²/g. On the other hand, a specific surface area of a pore inthe cell wall 12 is 0.12 m²/g as measured by a mercury pressure process,and the pore has a surface area of about 50-100 times. This means thatwhen the alumina thin film 3 is formed on the surface of the cell wall 3at the same weight, if each surface of the SiC particles 4 constitutingthe cell wall 12 is individually covered as compared with the case ofsimply and uniformly covering the surface of the cell wall 12, thethickness of the alumina thin film 3 can be made to {fraction(1/50)}-{fraction (1/100)} for obtaining the same effect.

[0086] That is, when the alumina thin film 3 is uniformly formed underthe conventional technique such as wash coat, in order to coat aluminaof about 3 mass % required for the activation of the catalyst, thethickness of the alumina thin film 3 is required to be 50 μm. In thiscase, as the pressure loss, the resistance passing through the aluminathin film 3 increases in addition to the resistance ΔP3 passing throughthe cell wall 2. Furthermore, the opening is small and ΔP1 becomeslarge. For this end, the pressure loss is considerably large as comparedwith the filter not coated with the alumina, and this tendency becomesmore remarkable when the particulate is deposited on the filter.

[0087] In this point, when the alumina is coated onto the surfaces ofthe SiC particles 4 constituting the cell wall 12 as in the ceramicsupport 15 used in the invention, in order to form the alumina thin filmof about 3 mass % required for the activation of the catalyst, thethickness is about 0.5 μm at maximum. In this case, as the increase ofthe pressure loss, the resistance ΔP3 slightly increases, but the otherpressure losses are substantially ignored and hence the pressure lossproperty is considerably improved as compared with the alumina layerformed by the conventional wash coat method.

[0088] (Heat Resistance of Alumina Thin Film 3)

[0089] In general, alumina has a higher specific surface area and issuitable as a film holding the catalyst. Particularly, it is desired todevelop the catalyst holding filter 10 stably operating at a highertemperature and having a high heat resistance at the present time, sothat the alumina thin film 3 is also required to have a more higher heatresistance.

[0090] In this point, according to the embodiment of the invention, (1)the shape of each alumina particle is rendered into a fine fiber, and(2) a rare earth oxide such as ceria (cerium oxide) or the like isincluded in order to improve the heat resistance of the alumina.

[0091] Particularly, contact points between alumina particles can bedecreased by adopting the construction of the former (1), and the graingrowth is controlled through the lowering of the sintering rate andhence the specific surface area can be increased to improve the heatresistance.

[0092] That is, the alumina thin film 3 covering the each surface of theSiC particles 4 in the ceramic support 15 according to the invention,the micro-sectional shape of each alumina particle indicates a hairedstructure of foresting small fibers. Therefore, mutually contact pointsbetween adjoining alumina small fibers are decreased to considerablyimprove the heat resistance.

[0093] As to the latter (2), the heat resistance is improved by addingceria or the like. Because, a new compound is formed on the surface ofthe crystal grains constituting the alumina thin film 3 to provide aneffect of preventing the growth of the alumina grains.

[0094] Moreover, in the embodiment of the invention, Si is supplied fromSiC or SiO₂ existing on slight surface layer thereof in the heattreatment and serves as an action of shielding the mass transfer path toimprove the heat resistance. According to the inventors' studies, it hasbeen confirmed that when SiC is intentionally treated at a hightemperature to form an oxide film, the heat resistance is furtherimproved.

[0095] (Wash Resistance of Alumina Thin Film 3)

[0096] The particulate deposited on the surface of the cell wall 12 ismainly composed of carbon, which can be removed by oxidation through amethod such as combustion or the like. However, a substance leaving asan ash is existent after the combustion. Such a substance is an oxide ora sulfate of a compound of Ca, Mg, Zn or the like added in an engine oilfor serving as a neutralizing agent, a lubricant or the like. And also,a catalyst previously included in a fuel for combusting carbon such asCeO₂, CuO or the like is deposited together with the particulate. Theseashes deposit in the running of the vehicle over a long time andincrease the pressure loss of the filter, so that they are required tobe washed with a high pressure water or the like. In this case, the ashcan completely be removed by washing under a pressure of not less than30 kg/cm².

[0097] In this connection, in case of the conventional alumina thin filmformed on the surface of the cell wall 12 by the wash coat, a thick coatlayer is existent on the whole surface of the cell wall 12 throughphysical adsorption, so that it is frequently peeled off in the abovewashing. On the contrary, in the holding film (alumina thin film 3) usedin the embodiment of the invention, alumina is thinly and individuallycoated on each surface of the SiC particles 4 constituting the ceramicsupport 15. And also, Si is supplied from SiC constituting the ceramicsupport 15 and chemically bonded. As a result, it is at a state ofrigidly adhering to each of SiC particles. Therefore, the resistance towashing is high and the durability as a film is strong.

[0098] (Regeneration Property of Alumina Thin Film 3)

[0099] In the embodiment of the invention, the alumina thin film 3 isadded with a rare earth oxide such as ceria (CeO₂) or lanthana (La₂O₃)in an amount of 10-80 mass %, preferably 20-40 mass % based on Al₂O₃, inwhich these oxides are uniformly dispersed in the surface or inside ofthe alumina thin film 3.

[0100] When ceria or the like is added to the alumina thin film 3 (it isdesirable to add together with a catalyst such as Pt or the like), thesupply of oxygen into the exhaust gas is activated by the action ofceria adjusting an oxygen concentration to improve a combustion removingefficiency of “soot (diesel particulate)” adhered to the filter. Hence,the regeneration ratio of the catalyst holding filter 10 is considerablyimproved. And also, the durability of the catalyst holding filter 10 canbe improved.

[0101] That is, the rare earth oxide such as ceria or the like not onlyimproves the heat resistance of alumina but also adjusts the oxygenconcentration on the surface of the catalyst holding filer 10. Ingeneral, hydrocarbon and carbon monooxide included in the exhaust gasare removed by oxidation reaction and NOx is removed by reductionreaction, but the composition of the exhaust gas always changes betweenrich zone and lean zone in the fuel and hence an atmosphere acting onthe surface of the catalyst holding filter 10 violently changes. Ceriaadded to the catalyst is relatively low in the oxidation-reductionpotential between Ce³⁺ and Ce⁴⁺ and reversibly promotes the followingreaction.

2CeO₂⇄Ce₂O₃+½O₂

[0102] That is, as the exhaust gas becomes rich zone, the above reactionprogresses in a right direction to supply oxygen to the atmosphere,while as the exhaust gas becomes lean zone, the reaction progresses in aleft direction to occlude extra oxygen in the atmosphere. In this way,the oxygen concentration in the atmosphere is adjusted, so that theceria acts to widen a breadth of air-fuel ratio capable of efficientlyremoving hydrocarbon, carbon monooxide or NOx.

[0103]FIG. 12(a) explains a mechanism of oxidation rate of the aluminathin film 3 not added with ceria (CeO₂). On the contrary, FIG. 12(b)explains a mechanism of oxidation rate of the alumina thin film 3 addedwith ceria. As shown in these figures, the catalyst having no ceriaactivates oxygen in the exhaust gas to oxidize soot. This reaction ispoor in the efficiency because oxygen in the fluid should be activated.

[0104] On the other hand, in the catalyst having CeO₂, oxygen issupplied by the following reaction.

CeO₂⇄CeO_(2-x) +x/2O₂

[0105] That is, oxygen discharged into the atmosphere and oxygen in theexhaust gas are activated by the catalyst (noble metal) to react withsoot (carbon) to produce CO₂ (CeO_(2-x) is oxidized to return to CeO₂).And also, CeO₂ and soot directly contact with each other, so that evenif a quantity of oxygen discharged is small, the soot can efficiently beoxidized.

[0106] In this case, CeO₂ holds the catalyst (noble metal) to increaseOSC (oxygen storing capacity). Because, the catalyst (noble metal)activates oxygen in the exhaust gas and also activates oxygen on thesurface of CeO₂ in the vicinity of the noble metal to increase the aboveOSC.

[0107]FIG. 13 and FIG. 14 show experimental results on regenerationproperties of a catalyst holding filter 10 (invention embodiment)comprising Pt as a catalyst, CeO₂ as a cocatalyst and needle-shapedAl₂O₃ as a support material and a catalyst holding filters 10 ofPt/needle-shaped Al₂O₃ (comparative example) and Pt/Al₂O₃ (wash coat)for the effect of adding a rare earth oxide such as ceria or the like tothe alumina thin film 3. In this experiment, diesel particulate filteradhered with soot (DPF fill length: 150 mm) is placed in an electricfurnace and heated to 650° C., while a diesel engine of 1100 rpm and 3.9Nm is connected thereto, during which a change of a filter temperature(measured at a position of 145 mm from an inlet port) (FIG. 13) when anexhaust gas (350° C.) and regeneration (combustion) rate (ratio oftemperature rising AT to lapse time At, FIG. 14) are measured.

[0108] As shown in FIG. 13, the conventional example (catalyst coatlayer 2 formed by wash coat) produces a peak temperature at 50 sec-700°C. as O₂ becomes rate-determining, while the comparative example (noceria (cocatalyst)) produces a peak temperature at 80 sec-800° C. as O₂becomes rate-determining. In the invention example, a high peaktemperature produces at a faster rate of 45 sec-900° C., which shows ahigher oxidation removal efficiency of soot and a higher regenerationratio. This also clearly appears as a difference of regeneration(combustion) rate in FIG. 14.

[0109] Furthermore, FIG. 15 shows a comparison of the regenerationratio, from which it is clear that the effect of the invention example(ceria-containing catalyst) is remarkable. DPF filters the soot in theexhaust gas. Therefore, the soot deposits in the inside of the DPF. Theaction of removing the deposited soot is called as regeneration.Therefore, a ratio of regenerated soot weight to deposited soot weightis represented by a percentage, which is defined as a regenerationratio.

[0110] Moreover, as the rare earth oxide, it is more favorable to use acomposite oxide of, for example, a rare earth element and zirconium inaddition to the above single oxide (CeO₂). It is considered that theproperty of controlling the oxygen concentration can be improved byincluding zirconium oxide in the rare earth oxide through the control ofthe grain growth by the rare earth oxide.

[0111] The rare earth oxide forming the composite oxide with zirconiumis favorable to have a particle size of about 1-30 nm, more preferably2-20 nm. When the particle size is less than 1 nm, it is difficult toform the composite oxide, while when the particle size exceeds 30 nm,the particles are apt to be sintered and the surface area of theparticle becomes small and hence the contact area with the exhaust gasbecomes small and a problem of weakening the activity remains. Further,there is a fear of increasing the pressure loss in the passing of theexhaust gas.

[0112] (Structure of Alumina Thin Film 3)

[0113]FIG. 5 shows a comparison between an electron microphotograph(×20000) of the ceramic support 15 in which each surface of SiCparticles 4 is covered with the alumina thin film 3 and an electronmicrophotograph (×5000) of the support in which the surface of the cellwall 12 is uniformly covered with alumina film (conventional technique).It is apparently seen that needle-shaped (small fibrous) alumina isforested on the surface of each SiC particle 4, which indicates a hairedstructure as shown in FIG. 3(b).

[0114] Such a structure of the alumina thin film 3, i.e. crystalstructure of alumina thin film 3 formed by covering each surface of SiCparticles 4 contains at least one of γ-Al₂O₃, δ-Al₂O₃ and θ-Al₂O₃. Thesmall fiber protruded alumina constituting the alumina thin film 3 has adiameter of 2-50 nm, a length of 20-300 nm and a ratio of fulllength/diameter of 5-50. And also, the alumina thin film 3 having such astructure is favorable to have a thickness of not more than 0.5 μm and aspecific surface area of alumina of 50-300 m²/g based on alumina.

[0115] The above-mentioned thickness of the alumina thin film 3 is theaverage distance between each surface of SiC particles 4 and each edgeof the small fiber protruded alumina. Furthermore, the diameter of thealumina is more desirable to be 5-20 nm and the ratio of the fulllength/diameter is more desirable to be 10-30.

[0116] The reason why the properties of the small fiber protrudedalumina thin film 3 are restricted as mentioned above is due to the factthat when the length of the small fiber protruded alumina is less than20 nm, it is difficult to ensure the required specific surface area,while when it exceeds 300 nm, the structure becomes brittle. And also,when the diameter is less than 2 nm, it is equal to or smaller than thesize of the catalyst such as noble metal or the like and does not serveas a catalyst holding layer, while when it exceeds 50 nm, it isdifficult to ensure the desirable specific surface area. Furthermore,when the ratio of full length/diameter is less than 5, it is difficultto ensure the required specific surface area, while when it exceeds 50,the structure becomes brittle and there may be caused the breaking ofthe small fiber protrusion by the washing operation or the like.

[0117] Further, the reason why the specific surface area of the aluminathin film 3 is restricted as mentioned above is due to the fact thatwhen it is less than 50 m²/g, the sintering of the small fiber protrudedalumina excessively progresses and the durability becomes poor. On theother hand, when the specific surface area exceeds 300 m²/g, the smallfiber protruded alumina is too finer and does not serve as so-calledcatalyst holding layer or becomes structurally brittle. Moreover, apreferable specific surface area is a range of 50-200 m²/g.

[0118] Then, an amount of alumina thin film 3 as a holding film in theceramic support 15 is favorable to be 0.1-15 mass % at an alumina ratio.When it is less than 0.1 mass %, the effect of improving the heatresistance becomes small, while when it exceeds 15 mass %, the pressureloss increases and the filter function lowers. More preferably, it is1-4 mass %.

[0119] In the ceramic support 15, when the support is, for example,porous silicon carbide (SiC), the silicon content is favorable to be0.01-10 mass %. When the silicon content is less than 0.01 mass %, acapacity of supplying Si is lacking and the effect of improving the heatresistance is small, while when the silicon content exceeds 10 mass %,the strength of the honeycomb filter lowers. This silicon content isapplied to the other silicon-containing ceramic, which is preferably0.01-10 mass %, more preferably 0.01-5 mass %, particularly 0.01-2 mass%.

[0120] Since each surface of the SiC particles 4 is individually coveredwith the alumina thin film 3, the surface of the ceramic support 15indicates a state of completely covering with the alumina thin film(holding film) 3. On the such a ceramic support 15 is held a noble metalelement as a catalyst and an element selected from Group VIa and GroupVIII in the Periodic Table. Concretely, these elements include platinum(Pt), palladium (Pd), rhodium (Rh), nickel (Ni), cobalt (Co), molybdenum(Mo), tungsten (W), cerium (Ce), copper (Cu), vanadium (V), iron (Fe),gold (Au), silver (Ag) and so on.

[0121] Therefore, at least one element selected from Pt, Au, Ag, Cu as anoble metal, Mo, W as an element of Group VIa in the Periodic Table, Fe,Co, Pd, Rh, Ni as an element of Group VIII in the Periodic Table and V,Ce as an element other than the above in the Periodic Table or acompound thereof is held as a catalyst on the alumina thin film 3.

[0122] For example, binary alloy or ternary alloy based on a combinationof the above elements is used as the compound. These alloys areadvantageous to be used together with the rare earth oxide such as ceriaor lanthana acting as a cocatalyst as previously mentioned. Such acatalyst holding filter 10 is less in the deterioration by poison (leadpoison, phosphorus poison, sulfur poison) and small in the heatdegradation and hence it is excellent in the durability. Moreover, acompound based on a combination with the other element (oxide, nitrideor carbide) may be used in addition to the above alloy based on thecombination of the elements.

[0123] Incidentally, as the binary alloy, there are Pt/Pd, Pt/Rh, Pt/Ni,Pt/Co, Pt/Mo, Pt/W, Pt/Ce, Pt/Cu, Pt/V, Pt/Fe, Pt/Au, Pt/Ag, Pd/Rh,Pd/Ni, Pd/Co, Pd/Mo, Pd/W, Pd/Ce, Pd/Cu, Pd/V, Pd/Fe, Pd/Au, Pd/Ag,Rh/Ni, Rh/Co, Rh/Mo, Rh/W, Rh/Ce, Rh/Cu, Rh/V, Rh/Fe, Rh/Au, Rh/Ag,Ni/Co, Ni/Mo, Ni/W, Ni/Ce, Ni/Cu, Ni/V, Ni/Fe, Ni/Au, Ni/Ag, Co/Mo,Co/W, Co/Ce, Co/Cu, Co/V, Co/Fe, Co/Au, Co/Ag, Mo/W, Mo/Ce, Mo/Cu, Mo/V,Mo/Fe, Mo/Au, Mo/Ag, W/Ce, W/Cu, W/V, W/Fe, W/Au, W/Ag, Ce/Cu, Ce/V,Ce/Fe, Ce/Au, Ce/Ag, Cu/V, Cu/Fe, Cu/Au, Cu/Ag, V/Fe, V/Au, V/Ag, Fe/Au,Fe/Ag and Au/Ag.

[0124] As the ternary alloy, there are Pt/Pd/Rh, Pt/Pd/Ni, Pt/Pd/Co,Pt/Pd/Mo, Pt/Pd/W, Pt/Pd/Ce, Pt/Pd/Cu, Pt/Pd/V, Pt/Pd/Fe, Pt/Pd/Au,Pt/Pd/Ag, Pt/Rh/Ni, Pt/Rh/Co, Pt/Rh/Mo, Pt/Rh/W, Pt/Rh/Ce, Pt/Rh/Cu,Pt/Rh/V, Pt/Rh/Fe, Pt/Rh/Au, Pt/Rh/Ag, Pt/Ni/Co, Pt/Ni/Mo, Pt/Ni/W,Pt/Ni/Ce, Pt/Ni/Cu, Pt/Ni/V, Pt/Ni/Fe, Pt/Ni/Au, Pt/Ni/Ag, Pt/Co/Mo,Pt/Co/W, Pt/Co/Ce, Pt/Co/Cu, Pt/Co/V, Pt/Co/Fe, Pt/Co/Au, Pt/Co/Ag,Pt/Mo/W, Pt/Mo/Ce, Pt/Mo/Cu, Pt/Mo/V, Pt/Mo/Fe, Pt/Mo/Au, Pt/Mo/Ag,Pt/W/Ce, Pt/W/Cu, Pt/W/Cu, Pt/W/V, Pt/W/Au, Pt/W/Ag, Pt/Ce/Cu, Pt/Ce/V,Pt/Ce/Fe, Pt/Ce/Au, Pt/Ce/Ag, Pt/Cu/V, Pt/Cu/Fe, Pt/Cu/Au, Pt/Cu/Ag,Pt/V/Fe, Pt/V/Au, Pt/V/Ag, Pt/Fe/Au, Pt/Fe/Ag, Pt/Au/Ag.

[0125] In order to hold these catalysts on the alumina thin film 3,there are considered various methods. As a method advantageouslysuitable for the invention, impregnation method, evaporation dryingmethod, equilibrium adsorption method, incipient wetness method, spraymethod are applicable. Among them, the incipient wetness method isadvantageous. This method is a methof wherein an aqueous solutioncontaining a given amount of a catalyst is added dropwise to the ceramicsupport 15 and at a time of slightly and uniformly wetting the surfaceof the support (incipient) the impregnation of the catalyst into thepores of the ceramic support 15 is stopped and thereafter the drying andfiring are carried out. That is, it is carried out by adding thecatalyst-containing solution dropwise to the surface of the ceramicsupport 15 with a billet or an injection. A holding amount of thecatalyst is determined by adjusting the concentration in the solution.

[0126] The manufacturing method of the catalyst holding filter 10 isdescribed below.

[0127] A characteristic of the manufacturing method of the catalystfilter 10 according to the invention lies in that the alumina thin film3 containing the rare earth oxide is formed on the irregular surface ofthe silicon-containing ceramic support 15 by a sol-gel method.Particularly, the rare earth oxide-containing alumina thin film 3 isindividually coated on each surface of SiC particles 4 forming the cellwall 12 by the immersion of the solution. After the calcination, themicro-section structure of the alumina thin film 3 is changed into analumina thin film (holding film) 3 indicating such a haired structurethat small fibers of alumina dispersed with ceria or the like areforested, and then a given amount of a catalyst is adsorbed and fixed(held) onto the surface of the alumina thin film 3.

[0128] Next, (1) formation step of ceramic support 15 and (2) holdingstep of catalyst are explained in detail. (1) Coating of alumina thinfilm 3 onto silicon-containing ceramic support 15

[0129] a. Preliminary Treatment Step

[0130] In this step, a treatment of oxidizing by heating at 800-1600° C.for 5-100 hours is carried out for supplying an amount of Si requiredfor assisting chemical bond to alumina to each surface of SiC particles4. Of course, this step may be omitted if a sufficient oxide film isexistent on the surface of the SiC particle 4. For instance, SiCsintered body itself contains about 0.8 mass % of SiO₂. Further, SiO₂ isincreased for improving the heat resistance, which is desirable to heatin an oxidizing atmosphere at 800-1600° C. for 5-100 hours. When it islower than 800° C., the oxidation reaction hardly occurs, while when itexceeds 1600° C., the oxidation reaction excessively proceeds and thestrength of the filter lowers. A recommend condition is 1000-1500° C.and 5-20 hours. Under this condition, SiO₂ enough to supply Si can beformed on the surface and the pressure loss property is not damagedwithout substantially changing the porosity and pore size of the ceramicsupport 15.

[0131] b. Solution Impregnation Step

[0132] In this step is carried out a treatment that a solution of ametal compound containing aluminum and a rare earth element, forexample, an aqueous mixed solution f aluminum nitrate and cerium nitrateis impregnated into each surface of the SiC particles 4 constituting thecell wall 12 by a sol-gel method to coat a rare earth oxide-containingalumina thin film 3.

[0133] With respect to a solution of aluminum-containing compound in theaqueous mixed solution, an inorganic metal compound and an organic metalcompound are used as a starting metal compound. As the inorganic metalcompound, there are used Al (NO₃)₃, AlCl₃, AlOCl, AlPO₄, Al₂(SO₄)₃,Al₂O₃, Al(OH)₃, Al and so on. Among them, Al(NO₃)₃ and AlCl₃ areparticularly preferable because they are easily dissolved in a solventsuch as water, alcohol or the like and easy in the handling.

[0134] As the organic metal compound, there are metal alkoxides, metalacetylacetonates and metal carboxylates. Concretely, there areAl(OCH₃)₃, Al(OC₂H₃)3, Al(iso-OC₃H₇)₃ and so on.

[0135] On the other hand, as the solution of cerium-containing compoundin the aqueous mixed solution are used Ce(NO₃)₃, CeCl₃, Ce₂(SO₄)₃, CeO₂,Ce(OH)₃, Ce₂(CO₃)₃ and so on.

[0136] As the solvent of the mixed solution, at least one or moreselected from water, alcohol, diol, polyvalent alcohol, ethylene glycol,ethylene oxide, triethanol amine, xylene and the like is used.

[0137] As the catalyst in the preparation of the solution, hydrochloricacid, sulfuric acid, nitric acid, acetic acid, hydrofluoric acid may beadded. Further, in order to improve the heat resistance of the aluminathin film 3, at least one element selected from Ce, La, Pr, Nd, Ba, Ca,Li, K, Sr, Si and Zr or a compound thereof other than oxide (nitrate,chloride, sulfate, hydroxide or carbonate) may be added to the startingmaterial in addition to the rare earth oxide.

[0138] In the embodiment of the invention, Al(NO₃)₃ and Ce(NO₃)₃ may bementioned as a preferable example of the metal compound because they aredissolved in the solvent at a relatively low temperature and are easy inthe preparation of the starting solution. And also, 1,3-butane diol isrecommended as a preferable example of the solvent. A first reason ofthe recommendation is due to the fact that the viscosity is proper andit is possible to form a gel film of a proper thickness on the SiCparticle 4 at a gel state. A second reason is due to the fact that thissolvent forms a metal alkoxide in the solution and is easy to form ametal oxide polymer consisting of oxygen-metal-oxygen bond or aprecursor of a metal oxide gel.

[0139] An amount of Al(NO₃)₃ is desirable to be 10-50 mass %. When it isless than 10 mass %, alumina amount having a surface area sufficient tomaintain a catalyst activity over a long time can not be held, whilewhen it exceeds 50 mass %, a heat generation quantity is large in thedissolution and gelation is easily caused.

[0140] An amount of Ce(NO₃)₃ is favorable to be 1-30 mass %. When it isless than 1 mass %, the oxidation of soot can not be promoted, whilewhen it exceeds 30 mass %, the grain growth of CeO₂ after the firingoccurs.

[0141] On the other hand, a compounding ratio of Al(NO₃)₃ to Ce(NO₃)₃ isfavorable to be 10:2. Because, as Al(NO₃)₃ becomes rich, thedispersibility of CeO₂ particles after the firing can be improved.

[0142] A temperature in the preparation of the impregnation solution ofthe metal compound is desirable to be 50-130° C. When it is lower than50° C., the solubility of a solute is low, while when it excceds 130°C., the reaction rapidly proceeds to cause gelation and hence it can notbe used as an application solution. A stirring time is desirable to 1-9hours. When it is within the above range, the viscosity of the solutionstable.

[0143] As to the above cerium-containing metal compound (Al(NO₃)₃ andCe(NO₃)₃), ZrO(NO₃)₃ or ZrO₂ is used as a zirconium source for forming acomposite oxide or solid solution of zirconium in addition to the aboveexample. It is favorable that they are dissolved in water or ethyleneglycol to from a mixed solution and then the mixed solution isimpregnated, and thereafter the composite oxide is obtained throughdrying and firing steps.

[0144] In the invention, it is important that the above adjustedsolution of the metal compounds is penetrated into all pores beingspaces between SiC particles 4 in the cell wall 12. For this end, it isfavorable to adopt a method wherein the ceramic support 15 is placed ina container and the solution of the metal compounds is filled thereinand then deaeration is conducted, a method wherein the solution isflowed into the ceramic support 15 from one side thereof and deaeratedfrom the other side, or the like. In this case, as a deaerationapparatus, a vacuum pump or the like in addition to an aspirator may beused. By using such an apparatus can be pulled out air from the pores inthe cell wall 12 and hence the solution of the metal compounds can beuniformly applied onto each surface of the SiC particles 4.

[0145] c. Drying Step

[0146] This step is a treatment that volatile component such as NO₂ orthe like is removed through evaporation to change the solution into gelto thereby fix onto the surfaces of the SiC particles 4 and at the sametime an extra solution is removed, which is carried out by heating at120-170° C. for about 2 hours. When the heating temperature is lowerthan 120° C., the volatile component hardly evaporates, while when itexceeds 170° C., the thickness of the gelated film becomes ununiform.

[0147] d. Calcining Step

[0148] This step is a treatment that calcination is carried out toremove residual components and form an amorphous alumina thin film 3.Concretely, it is desirable to heat at a temperature of 300-500° C. Whenthe calcining temperature is lower than 300° C., the residual organicsubstance is hardly removed, while when it exceeds 500° C., Al₂O₃ iscrystallized and the small fibrous protruded boehmite can not be formedby a subsequent hot water treatment.

[0149] e. Hot Water Treatment Step

[0150] This step is a treatment that the calcined ceramic support 15 isimmersed in hot water for forming the alumina thin film 3 of theaforementioned structure inherent to the embodiment of the invention.Immediately after such a hot water treatment, the particles of theamorphous alumina thin film 3 are discharged into the solution at a solstate by defloculation action and boehmite particles produced byhydration aggregate into small fibrous protrusions and form a stablestate against defloculation.

[0151] That is, the rare earth oxide-containing alumina individuallyadhered to each surface of the SiC particles 4 by this hot watertreatment are forested in small fibers (needle-shaped particles) andindicate so-called haired structure to form coarse surfaces. Therefore,the alumina thin film 3 having a high specific surface area is formed.In general, the sintering of alumina mainly progresses by surfacediffusion, and the specific surface area violently decreases in thephase transfer into a-alumina. However, since silica is included in thealumina particle, it is considered that this silica fills in the poresite of alumina or moves to the surface of the needle-shaped particle inthe course of the heat treatment to suppress the surface diffusion orthe sintering between the particles. Therefore, a viscous flowingmechanism through the sintering from a contact point between theneedle-shaped particles is preferential in the initial sintering of theceramic support 15, while silica shuts off the substance-moving pathbetween the needle-shaped particles at the late stage to obstruct thetransfer into a-alumina and hence it is considered that the sintering isnot progressed to maintain a higher specific surface area.

[0152] The temperature in the hot water treatment is desirable to be50-100° C. When it is lower than 50° C., the hydration of the amorphousalumina thin film 3 does not proceed and the small fibrous protrudedboehmite is not formed, while when it exceeds 100° C., water evaporatesand the step can not be maintained over a long time. The treating timeis desirable to be not less than 1 hour. When it is less than 1 hour,the hydration of the amorphous alumina is insufficient.

[0153] d. Firing Step

[0154] This step is a treatment that the boehmite produced by hydrationis heated to form alumina crystal. A preferable firing temperature is500-1000° C., and a preferable firing temperature is 5-20. When thetemperature is lower than 500° C., the crystallization is not promoted,while when it exceeds 1000° C., the sintering excessively proceeds andthe surface area tends to be lowered.

[0155] (2) Holding of Catalyst

[0156] a. Solution Adjusting Step

[0157] On the surface of the ceramic support 15 is covered the rareearth oxide-containing alumina thin film (holding film) 3 having thehaired structure as shown in FIG. 3(b), and a catalyst such as Pt or thelike is held on the irregular surface of the alumina thin film 3. Inthis case, the holding amount of the catalyst is determined by dropwiseadding an aqueous solution containing Pt or the like to the ceramicsupport 15 so as to impregnate by an water absorbing amount thereof at astate of slightly wetting the surface.

[0158] For example, the water absorbing amount of the ceramic support 15means that when a measured value of water absorbing amount of drysupport is 22.46 mass % and a mass of this support is 110 g and a volumethereof is 0.163 l, the support absorbs 24.79/l of water.

[0159] As a starting substance of Pt is used, for example, a solution ofdinitrodianmine platinum nitrate ([Pt(NH₃)₂(NO₂)₂]HNO₃, Ptconcentration: 4.53 mass %). In order to hold a given amount of 1.7 g/lof Pt, it is sufficient to hold 1.7 (g/l)×0.163 (1)=0.272 g of Pt on thesupport, so that the solution of dinitrodianamine nitrate (Ptconcentration: 4.53%) is diluted with a distilled water. That is, aweight ratio X(%) of solution of dinitrodianamine (Pt concentration:4.53 mass %)/distilled water is calculated to be X=0.272 (Pt amountg)/24.7 (water content g)/4.53 (Pt concentration mass %) and is 24.8mass %.

[0160] b. Solution Impregnation Step

[0161] The above adjusted aqueous solution of a given amount ofdinitrodianamine nitrate is added dropwise to both end faces of theceramic support 15 at constant intervals with a pipette. For instance,the solution is added dropwise onto the one side face at constantintervals every 40-80 droplets, whereby Pt is uniformly dispersed andfixed onto the surface of the alumina thin film 3 covering the ceramicsupport 15.

[0162] c. Drying and Firing Steps.

[0163] The ceramic support 15 after the addition of the aqueous solutionis dried at 110° C. for about 2 hours to remove water. Thereafter, thesupport is placed in a desiccator and left to stand for 1 hour tomeasure an adhesion amount by means of an electron balance or the like.Then, the firing is carried out in N2 atmosphere at about 500° C. forabout 1 hour to conduct metallization of Pt.

[0164] The catalyst holding filter 10 according to the invention is usedas a filter for the purification of the exhaust gas. As one applicationexample of plain honeycomb ceramic support 15, there are an oxidationcatalyst for a gasoline engine, a ternary catalyst and an oxidationcatalyst for a diesel engine. As the other application, there is adiesel particulate filter alternately sealed into a checkered pattern ofthe honeycomb.

[0165] The diesel particulate filter (hereinafter abbreviated as DPFsimply) itself has only a function of catching particulate (floatedparticle matter: PM), but when the catalyst is held to the filter,hydrocarbon and carbon monooxide in the exhaust gas can be oxidized.

[0166] And also, when holding NOx selective reduction type catalystcomponent or occlusion type catalyst component capable of reducing NOxeven in an oxidizing atmosphere such as diesel exhaust gas, thereduction of NOx is possible. Moreover, the particulate caught in theDPF brings about its deposition and the increase of pressure loss ofDPF, so that it is usually required to regenerate by removing throughcombustion treatment or the like. A temperature of starting combustionof soot (carbon) being a main component of the particulate included inthe usual diesel exhaust gas is approximately 550-630° C. In this point,when the catalyst is held on the DPF, the combustion reaction pass ofsoot changes and the energy obstruction can be decreased. Hence, thecombustion temperature can largely be lowered to 300° C. and the energyrequired for the regeneration can be reduced and the DPF system having ahigher regeneration ratio can be constructed by a synergistic actionwith so-called above action of ceria.

[0167] As mentioned above, the catalyst holding filter 10 according tothe invention is said to be particularly favorable for application to adiesel exhaust gas treating system, but the following functions can beexpected.

[0168] A. Function as an Oxidation Catalyst for Diesel Exhaust Gas

[0169] (1) Function of purifying exhaust gas . . . oxidation of THC (allhydrocarbon), CO

[0170] (2) Function not obstructing operation of engine . . . pressureloss

[0171] B. Function as a Diesel Particulate Filter Provided with Catalyst

[0172] (1) Function of purifying exhaust gas . . . Combustiontemperature of soot, oxidation of THC, CO

[0173] (2) Function not obstructing operation of engine . . . pressureloss

EXAMPLES

[0174] The invention will be described in detail with reference toexamples and comparative examples below.

[0175] A first example is carried out for confirming action and effectwhen a ceria-containing alumina thin film 3 is held on a surface of aceramic support 15 having varied pore size and porosity.

[0176] Manufacturing methods of the examples and comparative examplesare shown in Table 1 in lumps.

[0177] Moreover, the manufacture of the ceramic support is based on thefollowing system. At first, silicon carbide powder having a relativelylarge average particle size (powder A) and silicon carbide powder havinga relatively small average particle size (powder B) and, if necessary,silicon carbide powder having a middle average particle size (powder C)are mixed as starting materials. Then, a spherical acryl resin (density1.1 g/cm³) as a pore-forming agent for forming objective porosity andpore size is mixed with the staring silicon carbide powder at a volumeratio. Further, methylcellulose as a forming assistant is mixed with thestarting silicon carbide powder at a weight ratio. Finally, a dispersionconsisting of an organic solvent and water is mixed at a weight ratioshown in Table 1 based on total of all starting materials. Thereafter,the mixed starting materials are milled and shaped into a honeycomb byextrusion shaping, and then a part of cells 11 is sealed into acheckered pattern, Next, the shaped body is dried at 150° C., degreasedat 500° C. and fired in an inert atmosphere at a firing temperature andfiring time shown in Table 1 to obtain a ceramic support of each exampleand comparative example. The average pore size is measured by a mercurypressure method. An average value m2 of the pore size, a porosity and astandard deviation SD2 of pore size distribution represented by a commonlogarithm of the pore size are shown in Table 1. TABLE 1 Form- ingFiring Po- Pore-forming assis- Disper- temper- Firing Pore ros- StandardPowder A Powder B Powder C agent tant sion ature time size ity deviationcomparative  8 μm 70% 0.5 μm 30% —  8 μm  0%  6% 15% 2200° C.  4 hr  8μm 35% 0.2 example 1 comparative  10 μm 70% 0.5 μm 30% — 10 μm  0%  6%16% 2200° C.  6 hr  10 μm 35% 0.2 example 2 comparative  60 μm 70% 1.0μm 30% — 60 μm  0%  6% 16% 2200° C.  8 hr  60 μm 35% 0.2 example 3comparative 100 μm 60% 5.0 μm 30% 30 μm 10% 100 μm  0%  6% 16% 2200° C.15 hr 100 μm 35% 0.2 example 4 comparative 250 μm 60% 5.0 μm 30% 50 μm10% 250 μm  0%  6% 18% 2200° C. 20 hr 250 μm 35% 0.2 example 5comparative  8 μm 70% 0.5 μm 30% —  8 μm  2% 10% 18% 2200° C.  4 hr  8μm 40% 0.2 example 6 Example 1  10 μm 70% 0.5 μm 30% —  10 μm  3% 10%18% 2200° C.  6 hr  10 μm 40% 0.2 Example 2  60 μm 70% 1.0 μm 30% —  60μm  3% 10% 19% 2200° C.  8 hr  60 μm 40% 0.2 Example 3 100 μm 60% 5.0 μm30% 30 μm 10% 100 μm  3% 10% 20% 2200° C. 15 hr 100 μm 40% 0.2 Example 4250 μm 60% 5.0 μm 30% 50 μm 10% 250 μm  5% 10% 20% 2200° C. 20 hr 250 μm40% 0.2 comparative  8 μm 70% 0.5 μm 30% —  8 μm  5% 13% 20% 2200° C.  4hr  8 μm 50% 0.2 example 7 Example 5  10 μm 70% 0.5 μm 30% —  10 μm  5%15% 22% 2200° C.  6 hr  10 μm 50% 0.2 Example 6  60 μm 70% 1.0 μm 30% — 60 μm  5% 18% 23% 2200° C.  8 hr  60 μm 50% 0.2 Example 7 100 μm 60%5.0 μm 30% 30 μm 10% 100 μm 10% 18% 23% 2200° C. 15 hr 100 μm 50% 0.2Example 8 250 μm 60% 5.0 μm 30% 50 μm 10% 250 μm 10% 20% 25% 2200° C. 20hr 250 μm 50% 0.2 comparative  8 μm 70% 0.5 μm 30% —  8 μm 15% 15% 29%2200° C.  4 hr  8 μm 70% 0.2 example 8 Example 9  10 μm 70% 0.5 μm 30% — 10 μm 18% 18% 30% 2200° C.  6 hr  10 μm 70% 0.2 Example 10  60 μm 70%1.0 μm 30% —  60 μm 20% 20% 30% 2200° C.  8 hr  60 μm 70% 0.2 Example 11100 μm 50% 5.0 μm 35% 30 μm 15% 100 μm 20% 20% 31% 2200° C. 15 hr 100 μm70% 0.2 Example 12 250 μm 50% 5.0 μm 40% 50 μm 10% 250 μm 20% 20% 31%2200° C. 20 hr 250 μm 70% 0.2 comparative  8 μm 70% 0.5 μm 30% —  8 μm20% 20% 33% 2200° C.  4 hr  8 μm 80% 0.2 example 9 Example 13  10 μm 70%0.5 μm 30% —  10 μm 20% 25% 33% 2200° C.  6 hr  10 μm 80% 0.2 Example 14 60 μm 70% 1.0 μm 30% —  60 μm 25% 30% 34% 2200° C.  8 hr  60 μm 80% 0.2Example 15 100 μm 60% 5.0 μm 30% 30 μm 10% 100 μm 25% 30% 35% 2200° C.15 hr 100 μm 80% 0.2 Example 16 250 μm 55% 5.0 μm 30% 50 μm 15% 250 μm25% 30% 35% 2200° C. 20 hr 250 μm 80% 0.2 comparative  8 μm 70% 0.5 μm30% —  8 μm 23% 40% 36% 2200° C.  4 hr  8 μm 85% 0.2 example 10comparative  10 μm 70% 0.5 μm 30% —  10 μm 23% 40% 36% 2200° C.  6 hr 10 μm 85% 0.2 example 11 comparative  60 μm 60% 1.0 μm 30% —  60 μm 30%40% 38% 2200° C.  8 hr  60 μm 85% 0.2 example 12 comparative 100 μm 60%5.0 μm 30% 30 μm 10% 100 μm 30% 40% 38% 2200° C. 15 hr 100 μm 85% 0.2example 13 comparative 250 μm 60% 5.0 μm 30% 50 μm 10% 250 μm 30% 40%40% 2200° C. 20 hr 250 μm 85% 0.2 example 14 Example 17  60 μm 90% 1.0μm 10% —  60 μm  0% 13% 21% 2200° C.  8 hr  60 μm 50% 0.01 Example 18 60 μm 70% 1.0 μm 30% —  60 μm 10% 20% 22% 2200° C.  8 hr  60 μm 50% 0.1Example 19  60 μm 50% 1.0 μm 30% 30 μm 20%  60 μm 20% 25% 22% 2200° C. 8 hr  60 μm 50% 0.4 Example 20  60 μm 50% 1.0 μm 30% 40 μm 20%  60 μm40% 25% 23% 2200° C.  8 hr  60 μm 50% 0.5

[0178] Then, 30 g/l of a catalyst coat layer is held on each of theceramic supports. Thereafter, 10 g/l of soot is caught.

[0179] Further, a gas is flowed at a flow rate of 10 m/s to measure apressure difference. The results are shown in FIG. 19(a).

[0180] From Examples 1-16 and Comparative Examples 1-14, it isunderstood that when 30 g/l of the catalyst coat layer is held on theceramic support and soot is caught for 5 hours, the pressure lossbecomes high at the pore size of not more than 10 μm, and as the poresize becomes large, the pressure loss gently rises from about 50 μm to250 μm, and considerably rises at more than 250 μm.

[0181] And also, when the porosity is not more than 35%, the pressureloss becomes very high. As the porosity becomes higher, the pressureloss becomes low, and the increase again occurs about 70%, and thepressure loss becomes high. When the cut wall face is observed, thepeeled and bridged catalyst coat layer is pointedly existent in thepores.

[0182] And also, the ceramic support in Examples 6, 17, 18, 19, 20 isconnected to an engine and catches soot for 5 hours, and a catchingefficiency is calculated from the caught amount. Next, the caught sootis washed and 30 g/l of a catalyst coat layer is held on each of theceramic supports.

[0183] Further, it is connected to the engine in the same manner asmentioned above and catch soot for 5 hours, and a catching efficiency iscalculated from the caught amount.

[0184] The results of the above catching efficiencies are shown in FIG.19(b).

[0185] From the results of the catching efficiency, it is understoodthat as the catalyst coat layer is held on the ceramic support, when thestandard deviation SD2 of the pore size distribution represented by acommon logarithm of the pore size is not more than 0.4, the catchingefficiency is improved by coating the catalyst, while when it exceeds0.4, the catching efficiency is degraded by coating the catalyst. Andalso, when the catalyst support is cut to observe the wall face, if thestandard deviation is not more than 0.4, the catalyst coat layer isuniformly existent, while when the standard deviation is more than 0.4,the catalyst coat layer is largely aggregated and is pointedly existenton places of the wall.

[0186] Subsequently, the ceramic support in Examples 6, 17, 18, 19, 20is heated at 300° C. in an electric furnace and thereafter a three-pointbending test is carried out every 20 supports. Then, an average offracture loads is taken. And also, the same bending test is conductedwith respect to the support held with 30 g/l of the catalyst coat layeraccording to the invention and the support coated on the wall asdescribed in JP-A-5-23512. The results are shown in FIG. 19(c).

[0187] As seen from this result, there is not caused the change in thestrength between the ceramic support before the coating of the catalystand the support coated on its wall with the catalyst. However, when thecatalyst is held on the sintered particle as in the invention, thestrength of the sintered neck portion is increased to improve thestrength. However, when adding heat of 300° C., if the standarddeviation of the pore size distribution is too low, the lacking of thestrength is caused due to slight cracks. If the standard deviationexceeds 0.40, the lowering of the strength becomes remarkable in aportion having a large pore size and finally the strength lowers.

[0188] As described in JP-A-5-23512, when the catalyst support isadjusted to a pore size of 15 μm and a porosity of 45% and coated on thesurface and soot is caught for 5 hours, the pressure loss is not lessthan 30 kPa, so that the catching for a long time can not be conductedwhen being coated on the wall.

Test Example

[0189] A test is carried out for confirming the action and effect on theceria-containing alumina thin film 3 formed on a surface of a ceramicsupport 15. A ceramic support 15 produced under conditions shown inTable 2 (Test Example 1, Comparative Test Examples 1, 2) is attached toa particulate filter (DPF) in an exhaust gas purification apparatus of adiesel vehicle to conduct a purification test. According to this testare examined pressure loss property, heat resistance and wash resistanceof the filter. The examined results are shown in the same table andFIGS. 7 and 8. TABLE 2 Comparative Comparative Test Test Test Example 1Example 1 Example 2 Honeycomb support SiC filter SiC filter SiC filterCell structure 14/200 14/200 14/200 Catalyst coat layer on particles oncell wall on cell wall Porosity of 45% 45% 45% ceramic support Pore sizeof ceramic 20 μm 8 μm 20 μm support Alumina diameter  10 nm — — thinlength 150 nm — — film length/ 15 — — diameter C_(e)O₂ (wt %) 30% — —Pressure loss 4.0 kPa 7.0 kPa 4.0 kPa property (before coat) Pressureloss 4.5 kPa 10.0 kPa 8.0 kPa property (after coat) Heat resistance Washresistance no peal at almost peal almost peal 70 kg/cm² at 10 kg/cm² at10 kg/cm².

[0190] (a) As shown in Table 2, before particulate (floating particulatematter: PM) is stored, Test Example 1 shows the pressure loss propertysubstantially equal to that having no alumina thin film 3, while thepressure loss when passing the same gas after the storing becomesconsiderably small as compared with those of Comparative Test Examples 1and 2.

[0191] (b) As shown in FIG. 7, Test example 1 is less in the lowering ofthe alumina specific surface area when being subjected to a heattreatment at the same temperature as compared with Comparative TestExample 1.

[0192] (c) As to the wash resistance, as shown in Table 1, Test Example1 is considerably superior to Comparative Test Examples 1 and 2.

[0193] (d) FIG. 15 shows a regeneration ratio (C content removed fromregeneration filter/C content adhered to filter prior regeneration). Incase of the alumina thin film 3 containing ceria, about 45% of C isremoved, while in case of the wash-coat alumina uniform film, only 20%of C is removed.

[0194] (e) As to the standard deviation SD1 of pore size distribution,as shown in FIG. 16, the average pore size m1 of the catalyst holdingfilter 10 in Test Example 1 is 30 μm as measured by a mercury pressuremethod. And also, the standard deviation SD1 of the pore sizedistribution represented by a common logarithm of the pore size is 0.30(se curve C1 in the graph of FIG. 16).

[0195] On the contrary, in the catalyst holding filter of Comparativetest examples 1, 2, the average value m2 of the pore size measured bythe mercury pressure method is 40 μm. And also, the standard deviationSD2 of the pore size distribution represented by a common logarithm ofpore size is 0.50 (curve C2 shown in FIG. 16). Therefore, the catalystholding filter 10 is rendered into a state of existing many pores havinga size suitable for catching the particulate, whereby the particulatecan surely be caught. Therefore, a filter 3 having a low pressure lossand a high catching efficiency can be realized.

Test Example 2

[0196] This example shows test results on various properties whenplatinum (Pt) as a catalyst is held on a ceramic support 15 in a dieselparticulate filter (DPF). The test conditions and properties are shownin Table 3. The results are shown in FIGS. 8, 9 and 10.

[0197] Moreover, this test example has an alumina thin film 3 (8 g/l) ona surface of SiC particle 4 in a ceramic support 15. Reference TestExample has not any holding film on the surface of the ceramic support15. In Comparative Test example 3, an alumina uniform film is formed onthe surface of the ceramic support 15 by wash coat. TABLE 3 ReferenceComparative Test Test Test Test Example 2 Example 2 Example ExampleHoneycomb support SiC-DPF SiC-DPF SiC-DPF SiC-DPF Cell structure 14/20014/200 14/200 14/200 Catalyst coat layer on on on cell on cell particlesparticles wall wall Porosity of ceramic support 45% 45% 45% 45% Poresize of ceramic support 20 μm 20 μm 8 μm 20 μm Al₂O₃ content   8 g/l   8g/l none   8 g/l CeO₂ content   2 g/l 1 g/l (CeO₂) none none 1 g/l(ZrO₂) Pt content 1.7 g/l 1.7 g/l none 1.7 g/l Pressure PM0 g/l (10m/sec) 1 1 1 1.45 loss PM0 g/l (10 m/sec) 1 1 1 1.45 Property Heatresistance of Al₂O₃ coat Initial equilibrium 420° C. 420° C. >480° C.440° C. combustion temperature property of equilibrium 9.2 kPa 9.2 kPa —1 kPa soot pressure Combustion equilibrium 420° C. 420° C. — >480° C.property of temperature soot after equilibrium 9.2 kPa 9.2 kPa — — agingpressure THC, CO conversion rate

[0198] (1) Pressure Loss Property

[0199] As shown in FIG. 8, when Test Examples 2, 3 are compared withReference Test Example and Comparative Test Example 3, Test Examples 2,3 show a pressure loss property substantially equal to that of ReferenceTest Example having no holding film and are considerably superior in theeffect to Comparative Test Example 3.

[0200] (2) Heat Resistance

[0201] As shown in FIG. 9(a) and FIG. 9(b), with respect to Test Example2 and Comparative Test Example 3, a change of specific surface area ofalumina thin film 3 heated at 1200° C. is compared with a change ofequilibrium temperature heated at 9000. As seen from this comparisonresult, the effect of the test example is considerably developed.

[0202] (3) Combustion Property of Soot

[0203] A performance of combusting soot adhered to the catalyst holdingfilter 10 is evaluated by-an equilibrium temperature testing method.That is, a diesel engine is placed in a testing apparatus and thecatalyst holding filter (DPF) 10 is inserted on the way of the exhaustpipe, and the running is started at such a state. With the lapse of therunning time, soot is caught on the DPF, so that the pressure lossincreases. In this case, as the exhaust temperature is raised by anymethod, a point balancing a rate of depositing soot with an oxidationreaction rate of soot (equilibrium temperature) appears at a certaintemperature, while a pressure at this point (equilibrium pressure) canbe measured. It can be said that the lower the equilibrium temperatureand equilibrium pressure, the better the catalyst holding filter 10.

[0204] As a method of raising the exhaust gas temperature in this test,an electric heater is inserted between the diesel engine and the DPF. Inthis method, engine revolution number and load can be made constant, sothat the composition of the diesel exhaust gas does not vary during thetest and the equilibrium temperature and equilibrium pressure can beensured accurately. As the test conditions, a steady run of the dieselengine having a displacement of 273 cc is carried out at a revolutionnumber of 1250 rpm under a load of 3 Nm, and a volume of the test filteris 0.17 liter (□ 34 mm×150 mm).

[0205] The above test results are shown in FIG. 10. In FIG. 10, anexample of ceramic support 15 holding no catalyst is Reference Testexample. As seen from FIG. 10, the filter temperature rises togetherwith the rise of the exhaust gas temperature, but an equilibrium pointis seen at about 500° C. When Test Example 2 is compared withComparative Test Example 3, the equilibrium temperature is 400° C. and410° C., respectively, and is slightly significant, but the equilibriumpressure is 11 kPa and 9.2 kPa and is improved by about 20%.

[0206] After aging is carried out in an oxidizing atmosphere of 850°C.-20 hours, when the similar test is conducted, the equilibriumtemperature and pressure are not substantially changed in Test Example2, while they are degraded to the same state of the case holding nocatalyst in Comparative Test Example 3.

[0207] (4) Purification Ratio of THC, CO

[0208] This property is a general method for evaluating the oxidationcatalyst, which examines a relation between purification of so-calledTHC (all hydrocarbon) into CO2 and water and a purifying temperature ofCO into CO2. As the conversion ratio at a low temperature becomeshigher, this property is said to provide an excellent catalyst system.As a measuring method, the filter is arranged in a path of an exhaustgas in an engine. The amounts of THC and CO before and after the filterare measured by an exhaust gas analyzing meter to determine apurification ratio to the temperature.

[0209] As shown in FIG. 11, Test Example 2 is superior in theperformance to Comparative Test Example 3 because the purificationtemperatures of CO, THC decrease about 30° C. This is considered thatsince the catalyst is uniformly dispersed into the SiC particles 4 ofthe cell wall 12 in this test example, the time of passing the exhaustgas through the cell wall 12 is clearly longer than the time of passingthrough the wash coat and as a result, a chance of adsorbing CO, THC onan active point of Pt is increased. In other words, since the catalystis unfiormly dispersed in the SiC particles 4, the contact area of theexhaust gas with the catalyst coat layer 2 cam be made large. Therefore,the oxidation of CO and HC in the exhaust gas can be promoted.

[0210] Moreover, the embodiment of the invention can be modified asfollows.

[0211] The ceramic material constituting the catalyst holding filter 10is not necessarily a porous body as in the embodiment, nor a honeycombstructure. That is, it is possible to select a network aggregate ofceramic fibers, a ceramic foam or the like as a filter constitutingmaterial.

[0212] The ceramic material constituting the catalyst holding filter 10is not necessarily limited to silicon carbide as in the embodiment, andis possible to select, for example, silicon nitride, cordierite, sialonor the like.

[0213] The catalyst may not be held on the surface of the porous ceramicmaterial constituting the catalyst holding filter 10.

[0214] As mentioned above in detail, according to the invention, thereis provided the catalyst holding filter being less in the pressure lossof the exhaust gas and improving the mechanical strength. And also,according to the invention, there is provided the catalyst holdingfilter having a higher catching efficiency of particulate included inthe exhaust gas.

1. A catalyst holding filter for purification of exhaust gas,characterized in that a catalyst coat layer is formed on particles of aceramic support having an average pore size of 10-250 μm and a porosityof 40-80%.
 2. A catalyst holding filter for purification of exhaust gas,characterized in that a catalyst coat layer comprising a catalyst, acocatalyst and a support material is formed on particles of a ceramicsupport having an average pore size of 10-250 μm and a porosity of40-70%.
 3. A catalyst holding filter according to claim 1 or 2, whereinthe catalyst contains an element selected from noble metal element,element of Group VIa of the periodic table and element of Group VIII ofthe periodic table.
 4. A catalyst holding filter according to any one ofclaims 1-3, wherein the cocatalyst is at least one element selected fromcerium (Ce), lanthanum (La), barium (Ba) and calcium (Ca) or a compoundthereof.
 5. A catalyst holding filter according to any one of claims1-4, wherein the support material contains at least one selected fromalumina, zirconia, titania and silica.
 6. A catalyst holding filteraccording to any one of claims 1-5, wherein the ceramic support issilicon carbide, silicon nitride, cordierite, mullite, sialon, silica orzirconium phosphate.
 7. A catalyst holding filter according to any oneof claims 1-6, wherein the ceramic support has a honeycomb structurehaving plural through-holes defined by cell walls.
 8. A catalyst holdingfilter according to claim 7, wherein the ceramic support has a checkeredpattern formed by alternately sealing both end portions with sealingbodies.
 9. A catalyst holding filter according to any one of claims 1-8,wherein the average pore size is measured by a mercury pressure processand a standard deviation of pore size distribution when the pore size isrepresented by a common logarithm is not more than 0.40.
 10. A catalystholding filter according to any one of claims 1-8, wherein the averagepore size is measured by a mercury pressure process and a standarddeviation of pore size distribution when the pore size is represented bya common logarithm is not more than 0.20.
 11. A catalyst holding filteraccording to claim 1, wherein the average pore size is set to 10-100 μmand the porosity of the ceramic support is set to 50-80%.
 12. A catalystholding filter according to claim 1 or 2, wherein the catalyst containsan element selected from cerium (Ce), copper (Cu), vanadium (V), iron(Fe), gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium(Rh), nickel (Ni), cobalt (Co), molybdenum (Mo) and tungsten (W).
 13. Acatalyst holding filter according to claim 7, wherein both end portionsof the ceramic support are sealed with sealing members in a checkeredpattern, and positions to be sealed differ between one end and the otherend of the ceramic support.
 14. An apparatus for purification of exhaustgas comprising a casing arranged in an exhaust path of an internalcombustion engine and a ceramic support arranged in the casing andremoving particulate included in an exhaust gas, characterized in thatan average pore size of the ceramic support is set to 10-250 μm and aporosity thereof is set to 40-70%, and a catalyst coat layer comprisinga catalyst, a cocatalyst and a support material is formed on particlesof the ceramic support.
 15. An apparatus for purification of exhaust gasaccording to claim 8, wherein a standard deviation (SD1) of pore sizedistribution is not more than 0.20.